1. Field of the Invention
The present invention relates generally to a wireless power receiver and a controlling method thereof, and more particularly, to a wireless power receiver that is adapted to receive wireless power based on a resonance system and a controlling method thereof.
2. Description of the Related Art
A mobile user equipment such as a cell phone or a PDA (Personal Digital Assistant) is generally driven by a rechargeable battery, and electric energy is supplied to the battery in order to recharge the battery, by using a separate recharging apparatus including separate contact terminals. The recharging apparatus and battery are electrically connected when the contact terminals are in contact with each other.
However, since such a contact charging system has contact terminals exposed to the outside, the contact terminals tend to become contaminated with foreign materials, which compromises the battery charge. In addition, the exposure to humidity further compromises the charging.
In order to solve the foregoing problems, wireless charging or non-contact charging has recently been developed, and is being utilized in many electronic apparatuses.
Such wireless charging system uses wireless power transmission/reception technology, which allows a battery to automatically charge by placement of the battery on a charging pad without connecting a cell phone to a separate charging connector. Examples of this wireless charging system are a wireless electric toothbrush or wireless electric shaver. This wireless charging system can enhance a waterproofing function by wirelessly charging electronic products and also enhance portability of the electronic apparatus because wire battery chargers are not required. Accordingly, the further development of such technologies is expected in the impending electric car era.
The wireless charging system roughly includes an electromagnetic induction system using coils, a resonance system using resonance, and a Radio Frequency (RF)/Micro Wave Radiation system which is performed by converting electric energy to a micro wave and transmitting the converted micro wave.
The electromagnetic induction system has been primarily used to this point, but success has been derived from recent experiments involving wireless transmission of electric power from a distance of a several tens of meters by using microwaves. Therefore, all electronic apparatuses is wirelessly charged in the near future.
A method for transmitting electric power by electromagnetic induction involves transmitting electric power between a first coil and a second coil. When a magnet moves on a coil, an induced current is generated. By using the induced current, a magnetic field is generated at a transmitting end, and an electric current is induced according to the change of the magnetic field and energy is generated at a receiving end. This phenomenon is referred to as an “electromagnetic phenomenon” and the electric power transmission method by using the same has excellent energy transmission efficiency.
In 2005, Professor Soljacic of the Massachusetts Institute of Technology (MIT) presented the resonance system, which enables electricity to be wirelessly transmitted to a charging unit even if the charging unit is several meters away, using resonance electric power transmission system by the Coupled Mode Theory. The wireless charging system of a research team in MIT uses a physics concept of resonance, in which when a tuning fork is sounding, a neighboring wine glass sounds at the same frequency. Instead of resonating sound, the research team resonated electromagnetic waves including electric energy. The resonating electric energy is directly transferred only when there is an apparatus having a resonant frequency, and unused parts do not spread in the air but are reabsorbed into electromagnetic fields. Therefore, unlike other electromagnetic waves, it is believed that there is no influence on neighboring machines or human bodies.
The wireless power receiver according to the conventional resonance system includes a rectifying circuit that converts received Alternating Current (AC) waveforms to Direct Current (DC) waveforms and a DC-DC converting circuit that adjusts the rectified electric power with DC waveforms to a predetermined voltage value of the output terminal. However, since the DC-DC converting circuit necessarily uses a passive element having a large external value, efficiency is compromised. In addition, the DC-DC converting circuit is an externally-packaged active element, so operation is generally performed in several MHz or less, which is inefficient.
Accordingly, it is required to develop a wireless power receiver that can minimize usage of the DC-DC converting circuit.